Most prosthetic liners and sockets are static. A mold of the residual limb is made, and then the prosthetic liner and socket are designed around the mold from the day of the amputee patient's fitting for the mold. The reality is that residual limbs are continually changing. In fact, most amputees' residual limbs shrink over the course of any given day (typical case), much like people's foot size changes from morning to evening.
Current prosthetic liners use a flexible material, such as a thick layer of silicone or polyurethane based material, which help to hold the liner to the residual limb by its shape (fitted to patient), some suction, and the liner's elasticity. These liners are available with or without the distal locking feature and are usually worn with traditional prosthetic socks to allow for volume adjustments. As the limb shrinks over the course of a typical day, however, there often forms a significant gap between the liner and the hard shell of the socket of the prosthetic device, which can be addressed by the patent by adding more layers of cotton socks between the liner and the hard shell. If the fit becomes too tight, the patient then removes layers of cotton socks between the liner and the hard shell. This is time-consuming and cumbersome. If the patient fails to notice that the fit is becoming too loose or too tight, tissue damage can occur to the residual limb. Because the skin of the leg (below the knee amputee) and thigh (above the knee amputee) do not have many nerves compared to hands and feet, the patient often does not notice a poor fit until there is a problem, and in the case of slippage, even bleeding from abrasion, due to these areas of the body being relatively uninervated.
To address the maintenance of prosthetic liner and socket fit beyond a flexible liner, several strategies have been explored, such as a variety of suction and vacuum systems. Suction systems often consist of a soft liner equipped with a one-way valve and a sealing sleeve. The patient inserts his or her liner-covered limb into the socket and the application of body weight as he or she stands expels excess air through the valve. In a typical vacuum system, a sleeve creates a seal around the top edge of the socket, then a pump and exhaust valve remove virtually all air between the socket and the liner as the patient wears the device. This system regulates the vacuum level within a defined range. Benevolent Technologies uses a pump to pull vacuum around gelled beads to produce a form-fitting one-size-fits-all fit. Challenges from vacuum systems is that patients often don't like the feel and simply don't feel as secure using their prosthetic devices as compared to more traditional prosthetic liner systems. Vacuum systems provide a stronger fit than suction systems, but with vacuum systems, if the vacuum is too tight and restrictive, tissue damage can occur in the residual limb.
The modeling of a perfect fit prosthetic socket is complex and unique for each patient. The modeling for sockets, and test case uses, is currently being performed in Prof. Hugh Herr's Biomechatronics Laboratory at MIT, by Prof. Herr, the founder of iWalk, now Biom, with a custom fit socket (U.S. Ser. No. 13/836,835); however, this is a relatively static system with respect to fit. Humans are dynamic, particularly when in motion, and undergo marked changes from an initial prosthetic fitting, even with state-of-the-art modeling and design.
Smart materials have found used as sensors, such as using dielectric elastomeric actuators (DEAs) as sensors and self-sensors. Dielectric materials are poor conductors of electricity, but good at supporting electrostatic fields, so act as capacitors. SRI international, Artificial Muscle Inc., and Stretch Sense/University of Auckland have found that DEAs have the potential for sensing and for self-sensing, where self-sensing is sensing an electrical property of the actuator itself (U.S. Pat. Nos. 8,860,336, 7,521,840, 7,595,580, 6,768,246). The state of a DEA can be determined by sensing the capacitance between the electrodes. Due to the high voltages (kV range) applied to the electrodes which are necessary to actuate a DEA, implementing capacitive self-sensing is not as simple as applying the capacitive sensing techniques commonly applied in other fields. The methodology for self-sensing in DEAs is very complex.